In a fiber laser device used in a processing machine, a medical device and the like, an optical fiber amplifier for amplifying light generated by a seed light source such as a laser oscillator (MO: Master Oscillator) using an amplification optical fiber is typically used.
A double clad fiber having a core doped with active element such as rare earth element may be used as an amplification optical fiber. The double clad fiber may be a double clad fiber that allows only single mode light to propagate through a core thereof, or a double clad fiber that allows multi mode light to propagate through a core thereof. In the double clad fiber that allows only single mode light to propagate through the core, as the core generally has a small diameter, density of light propagating through the core may be too high in order to obtain a high laser output. In this case, energy of light is transferred to have a wavelength that is not desired due to a nonlinear optical effect so that an expected laser output cannot be obtained. Therefore, along with the recent demand for an optical fiber amplifier to have a high power, an optical fiber amplifier using a double clad fiber that allows multi mode light to propagate through a core thereof has received attention.
Patent Document 1 discloses that LP01 mode can be mainly amplified even by an amplification double clad fiber that allows multi mode light to propagate by providing a mode converter that excites only LP01 mode of light propagating through the core of the amplification double clad fiber that allows multi mode light to propagate. Further, Patent Document 1 suggests that LP01 mode can be efficiently amplified compared to a higher-order mode due to an effect called gain waveguide by using an amplification optical fiber, in which active element is doped into the central portion of a core of a double clad fiber and active element is not doped into the outer circumference portion of the core. Patent Document 2 discloses an idea for attenuating needless higher-order modes by using an amplification optical fiber, in which active element is doped into the central portion of a core of a double clad fiber and absorbing element which absorbs light is doped into the outer circumference portion of the core.    [Patent Document 1] U.S. Pat. No. 5,818,630    [Patent Document 2] U.S. Pat. No. 5,121,460